ON DIMORPHOUS BODIES: 193 
ive layers deposited around arhomboidal nucleus, possessed of 
positive double refraction. This refraction, however, is seen 
“to diminish in succeeding layers from a positive state till it 
disappears altogether ; beyond this neutral line it becomes ne- 
gative, and again gradually increases towards the boundaries of 
the crystal.’ * 
Diamond, topaz.—A similar observation has also been made 
by Sir David in regard to the diamond, which he found to con- 
sist occasionally of a succession of layers possessing different 
refractive powers and different densities ; and in the 2nd vol. 
of the Cambridge Transactions he has described the Brazilian 
topaz as consisting of “a central lozenge, surrounded with a 
border of a different kind, sometimes with additional varia- 
tions.” 
Traces of double refraction have also been observed by the 
same distinguished philosopher in many substances, the cry- 
stals of which, hitherto observed, belong only to the regular 
system, Among these are potash-alum, rock-salt, fluor-spar, 
and diamond. In connexion with the doctrine of dimorphism, 
these observations are all of value, not so much fro» the posi- 
tive information they give, as from their showing us what to 
look for. 
39. The conclusion we are at first sight inclined to draw from 
phznomena such as those above described, is, that such mine- 
rals, though to the eye homogeneous, are in reality made up of 
parts unlike in chemical constitution as they are in optical pro- 
perties ; and to this conclusion Sir David Brewster appears in- 
clined to give his assent. Mr. Whewell, in his report on 
Mineralogy}, thus expresses himself: “There would be some- 
thing utterly perplexing in this complexity in the structure of 
Objects apparently so simple, if we were to conceive such a kind 
of composition as formed of independent portions adhering to- 
gether; but we ought probably rather to conceive these rela- 
tions of parts as the result of a peculiar state of the equilibrium 
of the elastic ether which exists within the body, and on which 
its optical properties depend.” 
_ This explanation appears to apply very happily to optical 
differences exhibited by the several parts of a crystal as a whole, 
which disappear when it is broken into fragments, as is the 
case in the dodecahedral crystals of the sulphate of potash § ; 
* London and Edinburgh Phil. Mag., Sept. 1836, p. 166. 
+ Report of Meeting of the British Association at Liverpool. 
t Reports of British Association, vol.i. p. 340. 
§ Edinburgh Philosophical Journal, vol. i. p. 6. 
oO 
VOL. VI. 1837. 
